Document and settle on a commit message convention

[proppy]

Currently it seems that we use different conventions for public commit messages in this repository:

• @cdleary tend to use [Subsystem:Sub subsystem][downcase(Type of change)] Message
• @proppy tend to use subdirectory: message
• other contributors tend to use Message

In general Message start with an action verb like Add, Fix, Update or Improve.

It would be nice to settle on a unified convention like @cdleary's or re-use well established one like https://www.conventionalcommits.org/.

That would also help w/ things like #1380 and can also be enforced with pre-submit github actions.